As the refrigerant for a refrigerator, a chlorine-containing compound, such as a chlorofluorocarbon, has been widely used, but in consideration of the environment protection, it is being replaced by a compound containing no chlorine, such as a hydrofluorocarbon (HFC), and a natural refrigerant, such as carbon dioxide.
Examples of the hydrofluorocarbon in practical use include a saturated hydrofluorocarbon (which may be hereinafter referred to as a saturated HFC), which is represented by 1,1,1,2-tetrafluoroethane, difluoromethane, pentafluoroethane, and 1,1,1-trifluoroethane (which may be hereinafter referred to as R134a, R32, R125, and R143a, respectively).
For a car air-conditioner, however, it is currently difficult to use carbon dioxide requiring high pressure as the major component of a refrigerant, and the use of a saturated HFC having a high global warming potential is also being avoided.
Under the circumstances, it is being considered in recent years to use an unsaturated fluorinated hydrocarbon compound, such as HFO1234ze and HFO1234yf, as a refrigerant capable of being used in the current car air-conditioner, irrespective of the low global warming potential thereof. Examples of a base oil of a refrigerator lubricating oil capable of being combined with the refrigerant containing the unsaturated fluorinated hydrocarbon compound include an oxygen-containing compound, such as a polyoxyalkylene glycol compound, a polyol ester compound, and a polycarbonate compound (see PTLs 1 to 3).
An open type car air-conditioner as an example of the refrigerator uses a rubber hose for the piping thereof, and thus water tends to enter the system. Therefore, a polyoxyalkylene glycol compound is used rather than a polyol ester compound, which tends to undergo hydrolysis through contact with water, so as to form a carboxylic acid causing deterioration.
In the case of a car air-conditioner, the mixture of the refrigerator lubricating oil and the refrigerant (which is referred to as a mixed composition for a refrigerator) in the refrigerator is necessarily assumed to leak into the engine compartment due to the breakage of the refrigerator in an unanticipated situation, for example, a car accident.
The refrigerant, the use of which is being considered in recent years, such as HFO1234ze and HFO1234yf, has slight flammability and is used with the refrigerator lubricating oil containing the flammable polyoxyalkylene glycol compound, and therefore there is a possibility that the leaking mixed composition for a refrigerator ignites under the high temperature environment in the engine compartment in an unanticipated situation.
Accordingly, for a car air-conditioner, such a refrigerator lubricating oil is demanded that is capable of preventing autoignition under the high temperature environment in the engine compartment, i.e., has a high autoignition point.